Joe Patrick wrote:
I thought the existence of zones A, B, C & D was because, unlike Essex,
Bucks were too mean to contribute to the cost of the services on the Met.

Yes, that is the case IIRC also applying for Hertfordshire CC, serving
other places in the additional zones (Watford, et al.)

There are other examples of this elsewhere. Now Merseytravel controls
the Merseyrail system directly, they've replaced point-to-point seasons
with their Trioplus (all modes) and Railpass[1] (rail only) seasons,
which have a distinct set of (pricier) zones covering the outer reaches
of the system outside their normal area of jurisdiction.

[1] This was a recent introduction, and I personally feel it is a very
bad thing, as it'll encourage less "incidental" bus use and more
driving to stations and the likes.

Neil